This is just something I worked on for a bit since the slime blocks were released, so to be precise: from 30 April 2014 till 3 May 2014. Don't worry: I know there are tons of "3x3 slime doors" around, but this thing actually has some advantages I didn't see on Youtube (or anywhere else for that matter) before.

I put up the 100 meter sample of the elevator shown in the video as a download (using MCEdit schematics).